Search

A candidate gene approach to identifying novel causes of disease is concept-limiting and in the new era of high throughput sequencing there is now no need to restrict the experiment to a few interesting genes. We have recently completed a large-scale exon re-sequencing project using Sanger sequencing technology to analyse approximately 1 Mb of coding sequence of the X chromosome in probands from >200 families with various forms of intellectual disability. We review the lessons learnt from this experience. Comparing large data sets will certainly reveal pathogenic mutations in genes that were not possible to identify previously. However, the task of distinguishing pathogenic mutations from rare sequence variants is not easy and is the most substantial challenge to the next decade. High-throughput technology has the attraction of being cheap, fast and comprehensive but for projects that require detailed coverage of a genomic region at an exhaustive level they may require a combination of large-scale with a small-scale follow-up of difficult regions to sequence. The number of rare truncating variants present in coding regions of the X chromosome that are not pathogenic was 1%. The importance of the quality of the starting material both clinically and molecularly and the number of sequence variants both rare and common that any one individual has across their coding sequence is discussed.

Idiopathic infantile nystagmus (IIN) consists of involuntary oscillations of the eyes. The familial form is most commonly X-linked. We recently found mutations in a novel gene FRMD7 (Xq26.2), which provided an opportunity to investigate a genetically defined and homogeneous group of patients with nystagmus. We compared clinical features and eye movement recordings of 90 subjects with mutation in the gene ( FRMD7 group) to 48 subjects without mutations but with clinical IIN (non- FRMD7 group). Fifty-eight female obligate carriers of the mutation were also investigated. The median visual acuity (VA) was 0.2 logMAR (Snellen equivalent 6/9) in both groups and most patients had good stereopsis. The prevalence of strabismus was also similar ( FRMD7 : 7.8%, non- FRMD7 : 10%). The presence of anomalous head posture (AHP) was significantly higher in the non- FRMD7 group ( P < 0.0001). The amplitude of nystagmus was more strongly dependant on the direction of gaze in the FRMD7 group being lower at primary position ( P < 0.0001), compared to non- FRMD7 group ( P = 0.83). Pendular nystagmus waveforms were also more frequent in the FRMD7 group ( P = 0.003). Fifty-three percent of the obligate female carriers of an FRMD7 mutation were clinically affected. The VA's in affected females were slightly better compared to affected males ( P = 0.014). Subnormal optokinetic responses were found in a subgroup of obligate unaffected carriers, which may be interpreted as a sub-clinical manifestation. FRMD7 is a major cause of X-linked IIN. Most clinical and eye movement characteristics were similar in the FRMD7 group and non- FRMD7 group with most patients having good VA and stereopsis and low incidence of strabismus. Fewer patients in the FRMD7 group had AHPs, their amplitude of nystagmus being lower in primary position. Our findings are helpful in the clinical identification of IIN and genetic counselling of nystagmus patients.

Genetic abnormalities frequently give rise to a mental retardation phenotype. Recent advances in resolution of comparative genomic hybridization and genomic sequence annotation has identified new syndromes at chromosome 3q29 and 9q34. The finding of a significant number of copy number polymorphisms in the genome in the normal population, means that assigning pathogenicity to deletions and duplications in patients with mental retardation can be difficult but has been identified for duplications of MECP2 and L1CAM. Novel autosomal genes that cause mental retardation have been identified recently including CC2D1A identified by homozygosity mapping. Several new genes and pathways have been identified in the field of X-linked mental retardation but many more still await identification. Analysis of families where only a single male is affected reveals that the chance of this being due to a single X-linked gene abnormality is significantly less than would be expected if the excess of males in the population is entirely due to X-linked disease. Recent identification of novel X-linked mental retardation genes has identified components of the post-synaptic density and multiple zinc finger transcription factors as disease causing suggesting new mechanisms of disease causation. The first therapeutic treatments of animal models of mental retardation have been reported, a Drosophila model of Fragile X syndrome has been treated with lithium or metabotropic glutamate receptor (mGluR) antagonists and a mouse model of NF1 has been treated with the HMG-CoA reductase inhibitor lavastatin, which improves the learning and memory skills in these models.

The sequence of the HA1 subunit region of the haemagglutinin gene of influenza A/USSR/90/77, and A/Brazil/11/78, A/Lackland/3/78, A/England/333/80 and A/India/6263/80 was determined by dideoxy-sequencing methods using total virion RNA and specific oligonucleotide primers for reverse transcriptase. These 1977-1980 strains share a minimum of 85% amino acid sequence homology with influenza A/PR/8/34. Most of the surface amino acid substitutions which occurred during the evolution of A/PR/8/34 to A/USSR/90/77 and subsequently in the 1978-1980 strains are located in the 4 antigenic sites previously defined by an analysis of laboratory-selected mutants of A/PR/8/34. We deduce an evolutionary pathway for the 1977-80 strains and suggest their different epidemic properties may be a consequence of only a few amino acid changes.